Biomass combustion

ABSTRACT

The present invention relates to a means and method for at least injecting mitigant particles into the combustion region (fireball) of a biomass boiler. The mitigant particles mitigate the slagging, fouling and corrosion problems caused by biomass ash by at least capturing the biomass ash. The mitigant particles capture the biomass ash by forming a physical bond with the biomass ash such that it adheres to the surface of mitigant particles. By injecting the mitigant particles into the combustion region, the opportunity to capture biomass ash is optimised.

FIELD OF INVENTION

The present invention relates to the combustion of biomass fuel.

BACKGROUND TO THE INVENTION

In an attempt to reduce greenhouse gas emissions, biomass has become an increasingly important fuel source.

Biomass may be combusted in small or large scale-combustion systems. For example, biomass may be combusted in a pulverised fuel boiler (1) as depicted in FIG. 1. Biomass fuel is injected into the combustion region of a furnace (F) via burners (2). The heat generated by the combustion of biomass is used to produce superheated steam in superheaters and reheaters (3). The superheated steam is directed to rotate a turbine (not shown) and this, in turn, drives a generator (not shown) to produce electricity. In pulverised fuel boilers, combustion typically takes place at temperatures in the range of approximately 1200° C. to 1700° C.

During combustion, the biomass (B) releases solid residues of biomass ash (A) as shown in the scanning electron microscope image and corresponding schematic drawing of FIGS. 2 a and 2 b. If further heating takes place then alkali, alkali earth metals and/or transition element salts of the biomass ash may vaporise within the furnace. As shown in FIG. 1, the biomass ash forms vitreous and molten slag deposits on the heat exchange surfaces and walls within the furnace in a region designated S. Vitreous fouling deposits also form on the heat exchange surfaces and economiser surfaces (4) within the gas pass (GP). The fouling deposits may be formed by biomass ash that has been carried over from the furnace, by the condensation of alkali, alkali earth metals and/or transition element salts and/or by crystalline growth due to the sulfation of biomass ash. The slag and fouling deposits have a deleterious effect on the operation of the boiler. For example, the slag may flow down the furnace walls blocking the burners. The slag and fouling deposits act as a refractory on the heat exchange surfaces and so the heat transfer and efficiency of the boiler is diminished. The slag and fouling deposits are very difficult to remove using conventional removal methods. Hence, the slag and fouling deposits may only be removed during a forced outage period; which is undesirable. Also, metal chlorides within the slag and fouling deposits are highly reactive and they rapidly corrode the surfaces in the furnace and particularly in the gas pass.

The biomass ash components that contribute to slagging, fouling and corrosion (e.g. alkali, alkali earth metals, transition metal compounds in the form of oxides, sulphates, chlorides, silicates and/or phosphates) typically comprise particles that are sub micron in size (have a diameter less than 1 micron).

SUMMARY OF THE INVENTION

The present invention seeks to address or at least substantially counteract the slagging, fouling and/or corrosion problems associated with biomass combustion.

The present invention seeks to mitigate the slagging, fouling and corrosion problems caused by biomass ash by at least capturing the biomass ash. The present invention seeks to capture the biomass ash by adhering the biomass ash to the surface of mitigant particles. The present invention seeks the capture of biomass ash by at least injecting mitigant particles into the combustion (fireball) region of the furnace. By injecting the mitigant particles into the combustion region, the opportunity to capture biomass ash is optimised. Accordingly, the present invention is able to capture at least a substantial portion of the biomass ash released during combustion.

The present invention may further seek to mitigate the slagging, fouling and corrosion problems caused by biomass ash by forming friable slag and/or fouling deposits from agglomerations of the mitigant particles carrying the biomass ash, by scouring surfaces on which slagging and/or fouling and thereby corrosion can occur and/or by depleting one or more highly corrosive alkali metal chloride component of biomass ash.

The present invention is defined in the attached independent claims, to which reference should now be made.

In accordance with an aspect of the present invention, there is provided a method of mitigating the effects of slagging, fouling and/or corrosion caused by biomass ash in a biomass combustion system, the method comprising: injecting biomass into a combustion region of the biomass combustion system and combusting the biomass, whereby biomass ash is created,

and injecting a mitigant into the combustion region of the biomass combustion system, so as to capture at least some of the biomass ash.

The invention also provides a method of improving the combustion efficiency of a biomass combustion system, the method comprising injecting biomass into a combustion region of the biomass combustion system and combusting the biomass, whereby biomass ash is created, and injecting a mitigant into the combustion region of the biomass combustion system, so as to capture at least some of the biomass ash.

The method may comprise: injecting the mitigant and biomass into the combustion region using the same injection means.

Alternatively, or in addition, the method may comprise injecting the mitigant and the biomass into the combustion region using different injection means.

The method may comprise injecting the mitigant below the biomass.

The method may comprise injecting the mitigant above the biomass.

The method may comprise injecting the mitigant adjacent the biomass.

In one arrangement, the biomass fuel comprises a mixture of biomass and mitigant.

The biomass fuel may comprise a compound of biomass and mitigant.

In a preferred arrangement, the biomass and mitigant may be in pellet form.

Preferably, the mitigant comprises particles to which the biomass ash is able to adhere.

In a preferred arrangement, the method comprises injecting a mitigant, which mitigant comprises at least one of: pulverised fuel ash, ground granulated blast furnace slag, a crystalline material and an amorphous, glassy material.

The invention also provides a biomass combustion system comprising: a furnace having a combustion region; a gas pass; means for injecting biomass and a mitigant into the combustion region, whereby combustion of the biomass generates biomass ash and the mitigant comprises particles for capturing biomass ash.

The system may comprise one or more primary injection ports for injecting biomass into the combustion region and a one or more secondary injection ports for injecting the mitigant into the combustion region.

The system may comprise one or more injection ports for injecting a mixture or compound comprising the biomass and the mitigant into the combustion region.

In a preferred arrangement, the mitigant comprises aerodynamic particles for capturing biomass ash in the combustion region and/or in the gas pass.

At least some of the particles may have a deformation temperature such that at least a part of a surface layer of the particles becomes viscous when the particles are located in the combustion region.

In a preferred arrangement, the mitigant particles may have an average diameter greater than the average diameter of particles of the biomass ash.

The mitigant particles may be configured to form friable slag deposits on surfaces in the furnace and/or friable fouling deposits on surfaces in the gas pass.

The system may further comprise one or more air lances for removing the friable slag and/or friable fouling deposits.

The mitigant particles may be configured to scour surfaces in the furnace and/or the gas pass.

The mitigant particles may comprise sulphur for reacting with highly corrosive alkali metal chlorides of the biomass ash so as to form less corrosive sulphate salts.

Preferably the mitigant comprises one or more of: pulverised fuel ash, ground granulated blast furnace slag, a crystalline material and an amorphous, glassy material.

The invention also provides a biomass fuel for combustion in a biomass combustion system, the fuel comprising biomass and a mitigant for capturing biomass ash, whereby the mitigant comprises particles to which the biomass ash can adhere.

The biomass fuel may comprise a mixture of the biomass and the mitigant.

The biomass fuel may comprise a compound of the biomass and the mitigant.

Preferably, the biomass fuel is in pellet form.

The invention also includes a method of manufacturing a biomass fuel for combustion in a biomass combustion system, the method comprising:

mixing biomass and a mitigant for capturing biomass ash, whereby the mitigant comprises particles to which biomass ash can adhere.

The method may further comprise: mixing the biomass and mitigant in a storage means, during milling, prior to grinding and/or after grinding.

The method may further comprise: pelletizing the mixture of biomass and mitigant.

One aspect of the invention relates to means for injecting a mitigant for capturing biomass ash into a combustion region of a biomass boiler.

The means for injecting the mitigant may be arranged below, adjacent to and/or above corresponding means for injecting biomass into the combustion region.

The means for injecting the mitigant may be arranged to simultaneously inject the mitigant and biomass into the combustion region. The means for injecting the mitigant may inject a mixture composition and/or a compound composition comprising the mitigant and biomass.

Another aspect of the invention relates to a method for mitigating slagging, fouling and/or corrosion effects of biomass ash in a biomass boiler the method comprising:

injecting biomass into a combustion region of a biomass boiler, whereby the biomass generates biomass ash on combustion; injecting a mitigant for capturing biomass ash into the combustion region of the biomass boiler.

The method may further comprise: injecting the mitigant below, adjacent to and/or or above biomass injected into the combustion.

The method may further comprise: injecting the mitigant and biomass into the combustion region using the same injection means. Alternatively, the method comprises: injecting the mitigant and the biomass into the combustion region using different injection means.

A further aspect of the invention relates to a biomass combustion system comprising:

a furnace having a combustion region; a gas pass; and means for injecting biomass and mitigant into the combustion region, whereby the biomass releases biomass ash as it combusts and the mitigant comprises aerodynamic particles for capturing biomass ash.

The means for injecting may comprise one or more primary injection ports for injecting biomass into the combustion region and a one or more secondary injection ports for injecting the mitigant into the combustion region. The one or more secondary injection ports for injecting the mitigant may be arranged to inject the mitigant into the combustion region below, adjacent to and/or above the biomass.

The means for injecting may comprise one or more injection ports for injecting a mixture composition comprising the biomass and the mitigant into the combustion region.

The means for injecting may comprise one or more injection ports for injecting a compound composition comprising the biomass and the mitigant into the combustion region.

The mitigant particles at least capture biomass ash in the combustion region. The mitigant particles may capture one or more biomass ash components in the combustion region before the biomass ash components evaporate.

The mitigant particles are sufficiently aerodynamic to allow for free movement of the particles within the furnace and gas pass. As a result, the mitigant particles are also able to capture biomass ash in the gas pass. The mitigant particles may capture condensing biomass ash components in the gas pass. The mitigant particles may capture the sulfation of biomass ash in the gas pass.

The mitigant particles capture the biomass ash by physically bonding with the biomass ash such that the biomass ash is attached (adhered, arranged) to the surface of the mitigant particles.

The mitigant may comprise particles for securely capturing biomass ash. The mitigant may comprise particles for releasbly capturing biomass ash.

So as to improve the adhesion of the biomass ash to the particles in the combustion region, the mitigant particles preferably have a deformation point such that at least a part of the surface layer of the particle becomes viscous when the particles are located in the combustion region. Accordingly, when biomass ash and particles collide in the combustion region, the biomass ash adheres to the viscous surfaces and is physically captured by the mitigant particles.

The rate of producing a viscous surface depends on the carbon content of the mitigant particles. The mitigant particles may comprise carbon falling in the range of approximately 3% weight to 10% weight.

To enable capture, the mitigant preferably comprises particles with an average diameter that is larger than that of the biomass ash particles. Given that the biomass ash components that cause slagging, fouling and corrosion typically have a diameter of less than 1 micron, the mitigant particles preferably have an average diameter greater than 1 micron.

To enhance the capture of the biomass ash, the mitigant may comprise particles having a relatively high surface area. For example, the mitigant may comprise particles having a surface area in the range of approximately 1.0 m²/g to 2.0 m²/g.

The mitigant may comprise particles with a density that allows the mitigant to become entrained in the flue gas of the biomass combustion system. The mitigant may comprise particles having a particle density in the range of approximately 1.48 gcm⁻³ to 2.8 gcm⁻³.

Due to their aerodynamic characteristic, the mitigant particles are able to carry the captured biomass ash as they freely move in the biomass combustion system. The aerodynamic property of the mitigant particles is dependent on the size, shape and density of the particles.

The mitigant particles carrying the captured biomass ash may agglomerate on surfaces in the furnace and/or the gas pass forming slagging and fouling. By capturing the biomass ash, the slagging, fouling and corrosion features in the biomass combustion system are now predominantly/at least substantially regulated by the characteristics of the mitigant particles. Hence, the slagging, fouling and corrosion effects of the biomass ash are mitigated (diminished).

The mitigant particles are preferably configured such that, even when they are carrying biomass ash, they form slag and/or fouling deposits that are more friable than slag and fouling deposits of biomass ash. The slag and/or fouling deposits are preferably sufficiently friable to allow for their removal without requiring a forced outage period for cleaning. Accordingly, the biomass combustion system may comprise one or more air lances or other on-load cleaning devices, such as steam soot-blower, sonic horn, water cannon, water lance or controlled explosion, to remove slag and/or fouling deposits formed by an agglomeration of mitigant particles carrying captured biomass ash. Regular cleaning of the surfaces helps to regulate the build-up of deposits and thereby regulates the corroding effect of the deposits on the surfaces.

To further mitigate the problems of slagging, fouling and corrosion, the mitigant particles may be configured to provide a scouring effect on the surfaces of the furnace and/or gas pass, even when they are carrying biomass ash. The particles scour the surfaces so as to restrict, (minimise, reduce) the build-up of slag and/or fouling deposits and thereby minimise corrosion of the surfaces.

To further mitigate the corrosion effects of biomass ash, the mitigant may comprise particles that release sulphur in the combustion region. The sulphur may react with the alkali metal chlorides of the biomass ash. The reactions form sulphur salts that are less corrosive than chlorides. Hence, the slag and fouling deposits formed by the particles carrying the captured biomass ash are less corrosive.

The mitigant may be one or more from a list including, but not limited to: pulverised fuel ash, ground granulated blast furnace slag, a crystalline material or an amorphous, glassy material.

The biomass combustion system may combust one or more types of biomass. The biomass combustion system may at least substantially combust biomass fuel. The biomass combustion system may co-fire biomass fuel with one or more other fuels.

The biomass combustion system may be small scale or large scale. The biomass combustion system may be for commercial or industrial purposes. The biomass combustion system may be a pulverised fuel boiler, a fluidised bed boiler or any other suitable boiler.

A further aspect of the invention relates to a biomass fuel comprising biomass and a mitigant for capturing biomass ash, whereby the mitigant comprises particles to which the biomass ash can adhere.

The biomass fuel may comprise a mixture composition of the biomass and the mitigant.

The biomass fuel may comprise a compound composition of the biomass and the mitigant.

The biomass fuel may be in pellet form.

A further aspect of the invention relates to a method of manufacturing a biomass fuel comprising:

mixing biomass and a mitigant for capturing biomass ash, whereby the mitigant comprises particles to which biomass ash can adhere.

The method may comprise: mixing the biomass and mitigant in a storage means, during milling, prior to grinding and/or after grinding.

The method may comprise: pelletizing the mixture of biomass and mitigant.

The invention may include any combination of the features or limitations referred to herein, except such a combination of features as are mutually exclusive, or mutually inconsistent.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the present invention and to show how it may be carried into effect, reference shall now be made by way of example to the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a pulverised fuel boiler showing where slagging, fouling and corrosion due to biomass ash may occur;

FIGS. 2 a and 2 b are respectively a scanning electron microscope image and a corresponding schematic drawing showing how combusting biomass releases particles of biomass ash;

FIGS. 3 a and 3 b are respectively a scanning electron microscope image and a corresponding schematic drawing showing particles of pulverised fuel ash;

FIGS. 4 a and 4 b are respectively a scanning electron microscope image and a corresponding schematic drawing showing biomass ash captured on the surface of pulverised fuel ash particles;

FIG. 4 c is a scanning electron microscope image showing condensed potassium chloride salts, condensed potassium sulphate salts and calcium sulphate crystals (formed by sulfation) captured on the surface of the pulverised fuel ash particles;

FIGS. 5 a and 5 b are respectively a scanning electron microscope image and a corresponding schematic drawing showing an agglomeration of pulverised fuel ash particles carrying biomass ash;

FIGS. 6 a and 6 b are respectively a photograph and a schematic drawing showing friable and heavy slagging formed on the pendant chevrons of a superheater in a pulverised fuel boiler burning biomass with pulverised fuel ash;

FIG. 7 is a cross-sectional view showing the location of probes MP1 and MP2 in a pulverised fuel boiler burning biomass and pulverised fuel ash;

FIGS. 8 and 9 are graphs showing the corrosion rates measured at probe MP1 and MP2 respectively;

FIG. 10 is a table relating to the sodium and potassium content of samples of pulverised fuel ash and pulverised fuel ash carrying biomass ash collected from the pulverised fuel boiler depicted in FIG. 7;

FIG. 11 depicts a K-spectrum graph, elements table and scanning electron microscope image of a prewashed sample of pulverised fuel ash carrying biomass ash collected from the pulverised fuel boiler depicted in FIG. 7;

FIG. 12 depicts a K-spectrum graph, elements table and scanning electron microscope image of a washed sample of pulverised fuel ash carrying biomass ash collected from the pulverised fuel boiler depicted in FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a mitigant for controlling slagging, fouling and corrosion caused by the combustion of biomass.

The mitigant is configured to selectively target (regulate) the biomass ash components that generate undesirable slagging, fouling and corrosion effects (e.g. alkali, alkali earth metals, transition metal compounds in the form of oxides, sulphates, chlorides, silicates and/or phosphates).

The mitigation effects of the mitigant are preferably sufficient to reduce (restrict, limit) the slagging, fouling and corrosion caused by the biomass ash to acceptable operational levels. The mitigation effects of the mitigant also preferably lead to the formation of friable slagging and/or fouling that is easily removable.

By controlling the slagging, fouling and corrosion, the mitigant improves the efficiency of the heat transfer and operation of the biomass combustion system.

Injection of Mitigant

The mitigant is injected into a combustion (fireball) region of a biomass combustion system along with the biomass.

The mitigant may be injected into the combustion region using injection ports.

The mitigant and biomass may be separately injected into the combustion region. For example, the biomass may be injected into the combustion region using one or more primary injection ports and the mitigant may be injected using a one or more secondary injection ports. The one or more secondary injection ports for injecting the mitigant may be arranged below, adjacent to or above the one or more primary injection ports for the biomass. As a result, the mitigant may be injected below the fireball, into the fireball or onto the top of the fireball in the combustion region.

The mitigant and biomass may be injected into the combustion region using the same injection port(s). For example, a mixture composition or a compound composition comprising the mitigant and biomass may be injected into the combustion region using one or more injection ports.

The mixture composition may be formed by mixing the mitigant and biomass in a storage means, during milling, before or after grinding. For example, the mixture composition may be formed by blowing the mitigant into the biomass pipework prior to injection into the combustion region. The mixture composition may be formed by mixing the mitigant and biomass in storage silos. The mixture composition may be formed by metering mitigant and biomass into the mill or fuel feed system. The mixture may be formed by adding mitigant to the biomass conveying system. The mixture composition may be formed at the pellet plant by forming a pellet of mitigant and biomass.

For the purposes of this document, the terms “injection” and “injection means” and any variations of the words, should be interpreted as meaning any type of method and means that is suitable for inserting (transporting, introducing, presenting) the mitigant and/or biomass into the combustion region.

Mitigating Mechanisms

The mitigating mechanism of the mitigant can include any of:

-   -   capturing the biomass ash;     -   changing the form of the slag and fouling deposits;     -   the scouring of surfaces;     -   the depletion of highly corrosive alkali metal chlorides.

Capturing the Biomass Ash

The mitigant comprises particles for capturing the biomass ash.

The particles capture the biomass ash by physically bonding with the biomass ash such that the biomass ash is attached (adhered, arranged) to the surface of the particles. It is important to note that the capture of the biomass ash does not cause the mitigant to chemically react with the biomass ash. The particles may capture the biomass ash as the particles and biomass ash components collide together. The surface of the particles and/or the biomass ash may be sufficiently viscous (sticky, adhesive) so as to form a bond. The mitigant particles may capture the biomass ash by providing a supporting surface on which biomass ash components (by-products) can form (grow, collect).

Since the mitigant is injected into the combustion region of the furnace, the particles can at least capture biomass ash components in the combustion region. For example, the particles may capture one or more biomass ash components in the combustion region before the biomass ash components evaporate.

The mitigant is selected such that the particles are sufficiently aerodynamic to become entrained in the flue gas and thereby freely move within the furnace and gas pass of a biomass combustion system. As a result, the particles can capture biomass ash in the gas pass. For example, the particles may capture condensing biomass ash components in the gas pass. The particles may capture the sulfation of biomass ash in the gas pass.

By injecting both the mitigant and biomass into the combustion region the opportunity to capture the biomass ash is optimised. For example, the mitigant is able to capture biomass ash in both the combustion region and the gas pass. The mitigant is able to capture biomass ash in different phases. As a result, the mitigant is able to capture at least a substantial portion of the biomass ash.

The mitigant may securely capture the biomass ash such that the biomass ash is permanently bonded to the surface of the particles. The mitigant may releasbly capture the biomass ash such that it may be removed during processing or analysis.

The particles have a melt point and evaporation point above the gas pass temperature. The particles have an evaporation point above the combustion region temperature. The particles may have a melt point above that of the biomass ash. So as to improve the adhesion of the biomass ash to the particles in the combustion region, the particles preferably have a predetermined deformation temperature such that at least a part of the surface layer of the particles becomes viscous when the particles are located in the combustion region. Accordingly, when the biomass ash and particles collide in the combustion region, the biomass ash adheres to the viscous surfaces and is physically captured by the particles. The mitigant preferably comprises particles having a deformation temperature that falls within the temperature range of the combustion region. If the biomass combustion system is a pulverised fuel boiler then the mitigant may comprise particles having a deformation temperature that falls within the range of approximately 1130° C. and 1400° C. The mitigant may comprise particles having a deformation temperature that falls within the range of approximately 1130° C. to 1280° C.

The rate of producing a viscous surface depends on the carbon content of the mitigant particles. The mitigant particles may comprise carbon falling in the range of approximately 3% weight to 10% weight.

To aid capture, the mitigant preferably comprises particles with an average diameter that is larger than that of the biomass ash. Given that the biomass ash components that cause slagging, fouling and corrosion typically have a diameter of less than 1 micron, the particles preferably have an average diameter greater than 1 micron. The mitigant may comprise particles having diameters in the range of 1 micron to 100 micron. The mitigant preferably comprises particles having diameters in the range of 1 micron to 50 microns. The average diameter of the particles may fall within the range of 5 micron to 25 micron.

To enhance the probability of capture of the biomass ash, the mitigant may comprise particles having a high surface area. For example, the mitigant may comprise particles having a surface area in the range of approximately 1.0 m²/g to 2.0 m²/g.

The mitigant may comprise particles with a density that achieves sufficient capture and allows the mitigant to become entrained in the flue gas of the biomass combustion system. The mitigant may comprise particles having a particle density in the range of approximately 1.48 gcm⁻³ to 2.8 gcm⁻³. The mitigant may comprise a bulk density in the range of approximately 1.08 gcm⁻³ to 2.0 gcm⁻³.

Formation of Slag and Fouling Deposits

Due to their aerodynamic characteristic, the mitigant particles are able to carry the captured biomass ash as they freely move in the biomass combustion system.

The mitigant particles carrying the captured biomass ash may agglomerate on surfaces in the furnace and/or the gas pass forming slagging and fouling.

By capturing the biomass ash, the slagging, fouling and corrosion features in the biomass combustion system are now predominantly/at least substantially regulated by the characteristics of the mitigant. Hence, by capturing the biomass ash, the slagging, fouling and corrosion effects of the biomass ash are mitigated (diminished).

Therefore, the mitigant is selected such that the particles, even when carrying the captured biomass ash, preferably form slag and/or fouling deposits that are more friable than slag and fouling deposits of biomass ash. Hence, the slag and/or fouling deposits formed by the agglomeration of mitigant particles carrying the captured biomass ash are more easily removable. The slag and/or fouling deposits may be removed using any standard cleaning means and do not require a forced outage period for cleaning. Accordingly, the biomass combustion system may comprise one or more air lances (otherwise known as “soot blowers”) to remove slag and/or fouling deposits formed by an agglomeration of mitigant particles carrying captured biomass ash. The slag deposits may fall to the lower region of the furnace and collected as furnace bottom ash.

Regular cleaning of the surfaces regulates the build-up of deposits and thereby regulates the corroding effect of the deposits on the surfaces.

The friability of the slag and fouling deposits may be determined by factors such as the shape, size, density, surface area, melting point and/or evaporation point of the mitigant particles.

Scouring of Surfaces

To further mitigate the problems of slagging, fouling and corrosion the mitigant is selected such that the particles, even when carrying biomass ash, may provide a scouring effect on the surfaces of the furnace and/or gas pass. The particles scour the surfaces so as to restrict, (minimise, reduce) the build-up of slag and/or fouling deposits and thereby minimise corrosion of the surfaces.

The scouring effect is determined by factors such as the shape, size, surface area and/or density of the particles. The mitigant may comprise particles that are substantially spherical in shape so as to avoid undesirable erosion of the surfaces. Alternatively, the mitigant may comprise particles with an irregular shape. The irregular shaped particles of the mitigant may be graded so as to control (restrict) the eroding effect.

Depletion of Corrosive Alkali Metal Chlorides in Biomass

To further mitigate the corrosion effects of biomass ash, the mitigant may comprise particles that release sulphur in the combustion region. The sulphur may react with the alkali metal chlorides of the biomass ash, for example as shown below:

2NaCl_((s,l))+SO₂+O₂═Na₂SO₄+Cl₂

2KCl_((s,l))+SO₂+O₂═K₂SO₄+Cl₂

The reactions form sulphur salts that have a higher melting point and are less corrosive than chlorides. Hence, the slag and fouling deposits formed by the particles carrying the captured biomass ash are less corrosive.

The mitigant particles may comprise sulphur falling in the range of approximately 0.5% weight to 1.5% weight.

EXAMPLES

The mitigant may comprise pulverised fuel ash (PFA). Pulverised fuel ash is a by-product, formed during the combustion of pulverised coal, that is carried through the coal fired boiler by flue gasses and collected by electrostatic precipitators or other filtration apparatus. For this reason, PFA is otherwise referred to as “fly ash” or “flue ash”. To control pollution, pulverised fuel ash is typically captured and then stored, sold or disposed in landfills. Due to the substantial number of coal-fired boilers, PFA is available in large quantities. Recycling PFA to mitigate the undesirable slagging, fouling and corrosion effects of biomass ash advantageously reduces the storage/landfill cost and space.

Pulverised fuel ash is an amorphous, glassy material.

As shown in the scanning electron microscope image and corresponding schematic drawing of FIGS. 3 a and 3 b, pulverised fuel ash (PFA) comprises discrete, substantially spherical particles, which minimises the erosion of the heat exchange surface in the boiler whilst maximising the scouring effect.

Pulverised fuel ash has a narrow deformation temperature range of approximately 1130° C. to 1280° C., making the deposition behaviour predictable within the furnace and gas pass.

Pulverised fuel ash preferably comprises carbon falling in the range of 3% weight to 10% weight. As the PFA particles are heated in the combustion region of the furnace, the carbon combusts and the surface of the particles becomes molten and therefore viscous. Any biomass ash particles that collide with PFA particles adhere (stick) to the molten surface. Hence, the biomass ash particles are captured by the PFA particles. As a consequence, the PFA particles are able to capture higher % weight biomass ash components in the combustion region than in the gas pass.

Pulverised fuel ash has a fine particle size distribution range of approximately 5 micron to 100 micron. The PFA has a surface area of approximately 1.1 m²/g to 1.8 m²/g and density of approximately 1.48 gcm⁻³ to 2.8 gcm⁻³ so as to improve the probability for capture.

FIGS. 4 a and 4 b depict a scanning electron microscope image and corresponding schematic drawing showing small biomass ash (BA) particles captured on the surface of the larger PFA particles. FIG. 4 c depicts a scanning electron microscope image showing condensed potassium chloride salts and condensed potassium sulphate salts (X) and crystals of calcium sulphate (Y) captured on the PFA.

Pulverised fuel ash is chemically benign to the elements and compounds created by biomass combustion.

Due to the melting characteristics, particle size distribution, shape, surface area and density profile of pulverised fuel ash, agglomerations of PFA carrying biomass ash (AGLOM) form slag and fouling deposits that have a higher melting point and are heavier than biomass ash deposits (See FIGS. 5 a and 5 b). As a result, the slag and fouling deposits formed by the agglomeration of PFA carrying biomass ash are friable and can be easily removed using conventional cleaning methods such as air lances. Hence, a biomass combustion system will not require off-load cleaning when using PFA as a mitigant. For example, FIGS. 6 a and 6 b depict a photograph and a corresponding schematic drawing showing pendant chevrons (5) of a superheater in an upper region of a furnace of a pulverised fuel boiler burning pulverised biomass with pulverised coal ash. It can be seen that, due to the PFA, the slag deposits (SLAG) forming on the pendant chevrons are thick, heavy and friable deposits which can be easily removed using conventional cleaning methods without having to force an outage.

Pulverised fuel ash contains residual sulphur which is released in gas phase in the combustion region. The sulphur is sufficient (approximately 0.5% weight) to achieve conversion some of the highly corrosive chloride salts into less corrosive sulphur salts. Hence, the PFA reduces the corrosive effect of the biomass ash.

PFA can also reduce corrosion by capturing alkali metal, alkali earth metal or transition metal salts in the biomass ash prior to volatilisation of those salts. This action reduces the total amount of volatile metal salts available to contribute to corrosion.

In an alternative example, the mitigant may comprise ground granulated blast furnace (GGBF) slag. The slag may be graded to ensure it has a substantially consistent melting point that allows the surface of the slag particles to become viscous and less angular in shape in the combustion region. The slag may be graded to ensure it has appropriate dimensions and surface area profile.

The mitigant may comprise a mineral or mixture of minerals. The mitigant may be crystalline or an amorphous glassy material.

The quantity of mitigant required to provide a mitigating effect where the rate of slagging, fouling and corrosion rate are within acceptable rates and the slagging and/or fouling is easily removable (i.e. can be removed with forced outage) depends on the amounts of biomass ash created by the biomass during combustion, the chemical content of the biomass, the type of mitigating material and the configuration of biomass ash. For example, the ratio of mitigant to biomass may range from approximately 1.5% weight to 12% weight. Following tests, it has been found that for 350 t of biomass generating 0.5% ash, approximately 8.75 t of PFA is required to at least sufficiently mitigate the slagging, fouling and corrosion effects of the biomass ash to acceptable operational levels. For 350 t of biomass generating 1.0% ash, approximately 17.5 t of PFA is required to at least sufficiently mitigate the slagging, fouling and corrosion effects of the biomass ash to acceptable operational levels. For 370 t of high agricultural matter generating 2.2% or up to 3% or more of ash, approximately 40 t of PFA is required to at least sufficiently mitigate the slagging, fouling and corrosion effects of biomass ash to acceptable operational levels.

To test the mitigating effect of pulverised fuel ash, biomass and pulverised fuel ash were injected into the combustion region of a pulverised fuel boiler and burned at approximately 600 MW for 3 days. Different biomass fuels comprising wood and agricultural matter were tested. For example, biomass comprising wood and approximately 30% miscanthus. The biomass and PFA were injected separately into the combustion region.

To measure the slagging, fouling and corrosion rates in the pulverised fuel boiler, two measurements points in the pulverised fuel boiler were selected: MP1 in the furnace section and MP2 in the gas pass section as shown in FIG. 7. MP1 assessed the slag deposition and corrosion of the platen superheaters and reheaters in the furnace section. Hence, MP1 reflected the capture of biomass ash by PFA, prior to the evaporation of the biomass, in the furnace section and depletion of alkali metal chlorides by sulphur released from PFA. MP2 assessed the fouling deposition and corrosion of primary superheaters and economisers in the gas pass section. Hence, MP2 reflected the carryover of PFA with captured biomass ash, the capture of condensing alkali, alkali earth metals, transition element salts and/or oxide fumes by the PFA and sulfation in the gas pass section and the depletion of alkali metal chlorides by sulphur released from PFA.

The slagging, fouling and corrosion rates at MP1 and MP2 were measured using air cooled probes. The air cooled probes comprised specimen metal rings. The temperature of the specimens was maintained at 600° C. so that any volatile elements were forced to condense on the specimens.

As shown in the graphs depicted in FIGS. 8 and 9 the results from MP1 found that acceptable deposit build-up rates and low corrosion rates were achieved at MP1 when combusting biomass with PFA. No significant amounts of chlorine were found in the deposits on MP1. It was found that the highest corrosion rates occurred when burning agricultural matter with a high chlorine content e.g. straw. Due to the PFA, the rate of corrosion did not even appear to be affected when combusting biomass with 30% miscanthus.

The test results from MP1 and MP2 shows that PFA acts as a mitigating mechanism to reduce the slagging, fouling and corrosion caused by biomass ash within the pulverised fuel boiler.

To assess the biomass ash captured by the pulverised fuel ash, samples of pulverised fuel ash mitigant and fly ash from the precipitators of the pulverised fuel boiler (A2-1, A3-1, A4-2) were analysed using inductively coupled plasma mass spectrometry (ICP-MS).

The samples were prepared for analysis as follows:—

-   -   Take 100 ug of each sample     -   add 3 ml Nitric Acid to each sample     -   add 1 ml Hydrochloric Acid to each sample     -   add 1 ml of Hydrofluoric Acid to each sample     -   Follow by 1 hr in a microwave digester.     -   On removal add 1 ml of Boric acid and then return sample to the         microwave digester for 1 hr.     -   Wash a portion of each PFA mitigant and fly ash samples in         demineralised water (3 g in 10 ml) and shake for approximately         30 seconds. Allow the samples to settle and remove a portion of         the liquid fraction to create the samples “A2-1 washed”, “A3-1         washed”, “A4-2 washed” and “PFA mitigant washed”.

The prewashed and washed samples were then analysed on the ICP-MS to assess for leachable alkali metals such as sodium and potassium. These particular alkali metals are known to contribute to the undesirable slagging, fouling and corrosion problems of biomass gas.

As shown in the table of FIG. 10, the difference between the prewashed and washed fly ash samples A2-1, A3-1, A4-2, reflect the capture of sodium and potassium in soluble form in the gas pass. For example, sample A2-1 collected 2559.1 ppm Na-23 and 14846.4 ppm K-39 as it travelled through the gas pass.

The difference between the PFA mitigant and washed fly ash samples A2-1, A3-1, A4-2, reflect the capture of sodium and potassium in the furnace. For example, sample A2-1 collected 2151.658 ppm Na-23 and 7844.893 ppm K-39 as it travelled through the furnace.

Prewashed and washed fly ash samples were also analysed using scanning electron microscope imaging (SEM) and energy dispersive x-ray spectroscopy (EDX). The differences between the prewashed and washed fly ash samples as shown in the k-spectrum graphs, element tables and images of FIGS. 11 and 12 reflect the capture of soluble alkali metals in the gas pass. For example, the % weight of sodium in soluble form increased from 00.86 to 1.58 and the % weight of potassium in soluble form increased from 3.06 to 4.18 as the sample travelled through the gas pass. Moreover, the scanning electron microscope images in FIGS. 11 and 12 show how the pulverised fuel ash particles captured so much biomass ash in the gas pass that the spherical particles of the PFA appears to have disappeared in a “haze”.

Hence, the sample results show that PFA captures components of biomass ash and so act as a mitigating mechanism to reduce the slagging and fouling and thus corrosion caused by biomass ash within the pulverised fuel boiler.

The methods and apparatus described above may bring about an improved combustion efficiency, giving better oxidation and heat exchange in the furnace. A reduction in carbon monoxide and improved usage of oxygen are observed.

The introduction of the mitigant into the flame near to the seat of the combustion process produces an improvement in combustion quality. The mitigant may help to form an increased region of refractory, creating a more localised area of incandescence and radiated heat transfer promoting more complete combustion.

The mitigant may act in conjunction with the biomass ash to improve the dissociation of the biomass carbon structure, allowing improved char burn out and better use of oxygen. It may also catalyse or enhance the conversion of carbon monoxide to carbon dioxide.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance, it should be understood that the applicant claims protection in respect of any patentable feature or combination of features referred to therein, and/or shown in the drawings, whether or not particular emphasis has been placed thereon.

Throughout the description and claims of this specification, the words “comprise” and “contain”, and any variations of the words, means “including but not limited to” and is not intended to (and does not) exclude other features, elements, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context requires otherwise. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers or characteristics described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. 

1. A method of mitigating the effects of slagging, fouling and/or corrosion caused by biomass ash in a biomass combustion system, the method comprising: injecting biomass into a combustion region of the biomass combustion system and combusting the biomass, whereby biomass ash is created, and injecting a mitigant into the combustion region of the biomass combustion system, so as to capture at least some of the biomass ash.
 2. (canceled)
 3. The method according to claim 1 further comprising: injecting the mitigant and biomass into the combustion region using the same injection means.
 4. The method according to claim 1, further comprising injecting the mitigant and the biomass into the combustion region using different injection means.
 5. A method according to claim 4, comprising injecting the mitigant below the biomass.
 6. A method according to claim 4, comprising injecting the mitigant above the biomass.
 7. A method according to claim 4, comprising injecting the mitigant adjacent the biomass.
 8. A method according to claim 1, wherein the biomass fuel comprises a mixture of biomass and mitigant.
 9. A method according to claim 1, wherein the biomass fuel comprises a compound of biomass and mitigant.
 10. A method according claim 9 wherein the biomass and mitigant are in pellet form.
 11. A method according to claim 9 wherein the mitigant comprises particles to which the biomass ash is able to adhere.
 12. A method according to claim 1, wherein the mitigant comprises at least one of: pulverised fuel ash, ground granulated blast furnace slag, a crystalline material and an amorphous, glassy material.
 13. A biomass combustion system comprising: a furnace having a combustion region; a gas pass; means for injecting biomass and a mitigant into the combustion region, whereby combustion of the biomass generates biomass ash and the mitigant comprises particles for capturing biomass ash.
 14. The biomass combustion system according to claim 13, comprising a one or more primary injection ports for injecting biomass into the combustion region and a one or more secondary injection ports for injecting the mitigant into the combustion region.
 15. The biomass combustion system according to claim 13, comprising one or more injection ports for injecting a mixture or compound comprising the biomass and the mitigant into the combustion region.
 16. The biomass combustion system according to claim 13, wherein the mitigant comprises aerodynamic particles for capturing biomass ash in the combustion region and/or in the gas pass.
 17. The biomass combustion system according to claim 16, wherein at least some of the particles have a deformation temperature such that at least a part of a surface layer of the particles becomes viscous when the particles are located in the combustion region.
 18. The biomass combustion system according to claim 16, wherein the mitigant particles have an average diameter greater than the average diameter of particles of the biomass ash.
 19. The biomass combustion system according to claim 16, wherein the mitigant particles are configured to form friable slag deposits on surfaces in the furnace and/or friable fouling deposits on surfaces in the gas pass.
 20. The biomass combustion system according to claim 19, further comprising one or more air lances for removing the friable slag and/or friable fouling deposits.
 21. The biomass combustion system according to claim 16, wherein the mitigant particles are configured to scour surfaces in the furnace and/or the gas pass.
 22. The biomass combustion system according to claim 16, wherein the mitigant particles comprise sulphur for reacting with highly corrosive alkali metal chlorides of the biomass ash so as to form less corrosive sulphate salts.
 23. The biomass combustion system according to claim 13, wherein the mitigant comprises one or more of: pulverised fuel ash, ground granulated blast furnace slag, a crystalline material and an amorphous, glassy material.
 24. A biomass fuel for combustion in a biomass combustion system, the fuel comprising biomass fuel and a mitigant for capturing biomass ash, whereby the mitigant comprises particles to which the biomass ash can adhere.
 25. (canceled)
 26. (canceled)
 27. The biomass fuel according to claim 24 wherein the biomass fuel is in pellet form.
 28. (canceled)
 29. (canceled)
 30. (canceled) 